1. Field of the Invention
This invention relates to a laminate type dielectric device such as a laminate-type capacitor, a laminate-type piezoelectric actuator, etc, a production method, and an electrode paste material for forming an electode layer of the dielectric device.
2. Description of the Related Art
A laminate-type dielectric device produced by alternately laminating dielectric ceramic layers having various dielectric properties and electrode layers have been widely used in the past. Known electrode materials for forming these electrode layers include Pt, Pd, Ag, Ni, Cu, their mixtures and their alloys.
The problems encountered in producing the electrodes vary from electrode material to electrode material. Silver (Ag), for example, has a high dielectric constant and is relatively economical. However, Ag has a low melting point of 980° C. and is likely to invite migration and, therefore, has low reliability.
In contrast, palladium (Pd) is expensive but has a high melting point. Pd has therefore been used in the form of an Ag—Pd metal material to suppress migration and to raise the melting point of the electrode material (refer to Japanese Unexamined Patent Publication (Kokai) No. 5-304043).
The addition of Pd can suppress migration, it is true, but bonding is not sufficient between the electrode material and a ceramic material. Various measures have been taken to cope with this problem as described in Japanese Unexamined Patent Publications (Kokai) Nos. 5-304043 and 8-255509.
Nickel (Ni) involves the problems such as the occurrence of cracking and deformation and so-called “islanding” of the electrode due to superheating (rapid sintering of Ni at a temperature exceeding 1,000° C.). To cope with the problems, Japanese Unexamined Patent Publication (Kokai) No. 5-55077 proposes to mix Ni and NiO, and Japanese Unexamined Patent Publication (Kokai) No. 6-290985 proposes to add an oxide of a rare earth element.
Among them, the prior art technology using Ag aims at solving the problem that results from Ag, and the prior art technology using Ni aims at solving the problem of the superheating of Ni that is peculiar to Ni. However, these materials are expensive and, as long as these materials are used, it will be difficult to reduce the production cost of the laminate type dielectric device that has been widely used.
On the other hand, copper (Cu) could be a promising material among base metals as the economical electrode material. In connection with Cu-containing paste materials or electrodes, a technology is known that suppresses the occurrence of cracking resulting from oxidation expansion of Cu by mixing Cu and C2O in a suitable proportion (Japanese Unexamined Patent Publication (Kokai) No. 5-283274). Further, there are also known a method that forms a complex by using an organic phosphorus compound a metal (Cu) ion, and baking the complex to assist sintering of ceramics and to make the film thickness uniform (Japanese Unexamined Patent Publication (Kokai) No. 5-242724), and a method that suppresses the occurrence of warping and cracking of the device by limiting the Cu content to 40 to 70 wt % to reduce the coating thickness (Japanese Unexamined Patent Publication (Kokai) No. 5-234414).
Still another known technology uses a paste material containing 40 to 60 wt % of Cu powder having a mean particle diameter of 0.5 to 2 μm and a particle size distribution of 0.3 to 4 μm where the paste material is baked to a film thickness of 1 to 3 μm so as to (1) suppress the occurrence of voids between the device and the electrode and inside the electrode, (2) suppress the occurrence of breakage of the electrode resulting from warp of inner and outer electrodes, (3) prevent deformation of a ceramic component itself and (4) prevent inferior contact between the inner electrode and the outer electrode (Japanese Unexamined Patent Publication (Kokai) No. 5-190375). The methods and objects of using Cu electrodes are very diversified.
One of the prior art references that relates to the improvement of oxidation resistance of the Cu type electrode materials is Japanese Unexamined Patent Publication (Kokai) No. 7-230714. The technology described in this reference suppresses the oxidation of copper powder on the basis of the principle that, when a metallic boride is blended, boron is oxidized in preference to copper powder.
In contrast, the present invention that will be explained later does not prevent oxidation of the Cu electrode portion.
In order to obtain a laminate product between ceramic materials, particularly PZT type materials that will generically have a Pb(Zr,Ti)O3 type perovskite structure while a boundary portion remains under a satisfactory condition, it is preferred to simultaneously bake the laminate of the PZT type material and the Cu electrode material. However, the PZT type material is preferably baked in the oxidizing atmosphere because it is an oxide whereas the Cu electrode material is preferably baked in the reducing atmosphere, for example, because the Cu electrode material needs a high conductivity.
Adjustment of the baking atmosphere that simultaneously satisfies the requirements for the PZT type material and the Cu electrode material is not impossible. Even when such an adjustment of the atmosphere is realized, however, the problem yet remains unsolved as to whether or not the atmosphere condition can sufficiently exploit performance of the laminate product. For example, when the atmosphere condition is such that the Cu electrode material can be sufficiently reduced, the PZT type material is presumably reduced to a certain extent, too. Needless to say, the performance of PZT drops if the PZT type material is reduced.
The condition that permits sufficient reduction of the Cu electrode material may well be the condition that permits also PZT to sufficiently exhibit its performance. If the drop of performance is restricted for the secondary factor that bonding can be sufficiently maintained, for example, a drop of in performance results from partial reduction.
In other words, according to the prior art technologies, reduction partially deteriorates the piezoelectric property, partial contraction (expansion) invites the occurrence of de-lamination and curving and a drastic drop in performance. In contrast, according to the present invention, the demerit is only the drop of performance by partial reduction.
Performance of PZT may be improved by gradually weakening the reducing condition during the process, though the Cu material is sufficiently reduced once, so as to mitigate the degree of reduction of PZT.